1. Field of the Invention
The present invention relates to an image forming device such as a printer or a copy machine, and more particularly to an image forming device that transports cuts sheets one at a time and that includes a transport pathway switching mechanism that guides the cut sheets selectively toward a sheet-discharge pathway and toward a sheet turn-over/resupply transport pathway.
2. Description of Related Art
There has been known a conventional printer capable of printing on both sides of the same sheet. The printer includes a transport pathway switching unit for switching transport pathway of sheets. The pathway switching unit is located downstream from an image fixing unit and other image forming components for forming an image on one side of a sheet. The pathway switching unit selectively guides sheets from the image fixing unit toward either a discharge pathway or a return pathway. The discharge pathway leads to a stacker or other such post-image-formation processing unit. The return pathway leads back toward the image forming section of the printer so that an image can be formed on the other side of the sheet.
FIGS. 1 and 2 show a sheet transport switching mechanism used in a conventional image forming device. A sheet transport pathway 112 connects a fixing unit (not shown) with a return pathway 114 and a sheet discharge pathway 113. The return pathway 114 connects with the sheet transport pathway 112 at a divergence point P. The return pathway 114 is used to guide sheets back to the image forming section during two-side printing. A pivot shaft 152 is provided in the sheet transport pathway 112 at a position downstream from the divergence point P. A path gate 150 extends from the pivot shaft 152 toward an upstream side of the transport pathway. It should be noted that the divergence point P is located at the same position as the free end of the path gate 150. Also, sheet guides 135, 136, 137, and 138 are fixed to a main casing (not shown) and define the sheet transport pathway 112 and the return pathway 114.
FIG. 1 shows the path gate 150 pivoted counterclockwise for guiding sheets toward the return pathway 114. FIG. 2 shows the path gate 150 pivoted clockwise for guiding sheets toward the sheet discharge pathway 113, which leads to a stacker or other post-image forming unit (not shown). A gate arm 158 is formed integrally with the pivot shaft 152 at the opposite side of the pivot shaft 152 than the path gate 150. In order to pivot the path gate 150 either forward or backward, a stepping motor 154 is rotated a predetermined number of steps in the corresponding direction. Rotation of the stepping motor 154 is transmitted to a gate cam gear 156 through a motor gear 155. A cam roller 157 attached to the gate cam gear 156 rotates accordingly. A pulling spring 159 urges the gate arm 158 into abutment with either the cam roller 157 as shown in FIG. 1 or a stopper 156A as shown in FIG. 2 depending on the rotation angle of the gate cam gear 156.
When the stepping motor 154 rotates the gate cam gear 156 clockwise (with respect to the view of FIG. 1) by a predetermined amount from the position shown in FIG. 2, then the cam roller 157 moves downward into the position shown in FIG. 1. As a result, the gate arm 158 pivots counterclockwise and the path gate 150 moves into a posture for blocking the sheet discharge pathway 113 and guiding sheets toward the return pathway 114. A sheet sensor 153 is provided upstream from the divergence point P. The sheet sensor 153 detects when the trailing edge of a sheet passes by the sheet sensor 153. A calculation unit of a control system (not shown) uses this detection to calculate when the trailing edge of the sheet will pass by the path gate 150.
The path gate 150 is maintained in the position shown in FIG. 1 if the next sheet from the sheet transport pathway 112 is also to be sent to the return pathway 114. However, if the next sheet is to be sent to the sheet discharge pathway 113, then rotation of the stepping motor 154 is reversed when the trailing edge of the preceding sheet is determined to have passed by the path gate 150. When the stepping motor 154 rotates in the opposite direction, the gate cam gear 156 also rotates in reverse and the gate arm 158 pivots clockwise as viewed in FIG. 2 until the gate arm 158 abuts against the stopper 156A. At this time, the path gate 150 also pivots clockwise into the position shown in FIG. 2 for blocking the entrance to the return pathway 114 and for guiding sheets toward the sheet discharge pathway 113. The path gate 150 will have pivoted into the position shown in FIG. 2 before the subsequent sheet reaches the path gate 150, thereby opening up the newly selected transport pathway and blocking the other transport pathway so that the subsequent sheet can be guided toward the newly selected transport sheet.